Discourses
by
Thomas H. Huxley
Part 4 out of 5
prefisso e determinato tempo; il quale arrivato escan fuora a godere il
sole.
"Io m' immagino, che questo mio pensiero non vi parra totalmento un
paradosso; mentro farete riflessione a quelle tanto sorte di galle, di
gallozzole, di coccole, di ricci, di calici, di cornetti ed i lappole,
che son produtte dalle quercel, dalle farnie, da' cerri, da' sugheri, da'
leeci e da altri simili alberi de ghianda; imperciocche in quello
gallozzole, e particolarmente nelle piu grosse, che si chiamano coronati,
ne' ricci capelluti, che ciuffoli da' nostri contadini son detti; nei
ricci legnosi del cerro, ne' ricci stellati della quercia, nelle galluzze
della foglia del leccio si vede evidentissimamente, che la prima e
principale intenzione della natura e formare dentro di quelle un animale
volante; vedendosi nel centro della gallozzola un uovo, che col crescere
e col maturarsi di essa gallozzola va crescendo e maturando anch' egli, e
cresce altresi a suo tempo quel verme, che nell' uovo si racchiude; il
qual verme, quando la gallozzola e finita di maturare e che e venuto il
termine destinato al suo nascimento, diventa, di verme che era, una
mosca.... Io vi confesso ingenuamente, che prima d'aver fatte queste mie
esperienze intorno alla generazione degl' insetti mi dava a credere, o
per dir meglio sospettava, che forse la gallozzola nascesse, perche
arrivando la mosca nel tempo della primavera, e facendo una piccolissima
fessura ne' rami piu teneri della quercia, in quella fessura nascondesse
uno de suoi semi, il quale fosse cagione che sbocciasse fuora la
gallozzola; e che mai non si vedessero galle o gallozzole o ricci o
cornetti o calici o coccole, se non in que' rami, ne' quali le mosche
avessero depositate le loro semenze; e mi dava ad intendere, che le
gallozzole fossero una malattia cagionata nelle querce dalle punture
delle mosche, in quella giusa stessa che dalle punture d'altri animaletti
simiglievoli veggiamo crescere de' tumori ne' corpi degli animali."]
It is of great importance to apprehend Redi's position rightly; for the
lines of thought he laid down for us are those upon which naturalists
have been working ever since. Clearly, he held _Biogenesis_ as against
_Abiogenesis;_ and I shall immediately proceed, in the first place, to
inquire how far subsequent investigation has borne him out in so doing.
But Redi also thought that there were two modes of Biogenesis. By the one
method, which is that of common and ordinary occurrence, the living
parent gives rise to offspring which passes through the same cycle of
changes as itself--like gives rise to like; and this has been termed
_Homogenesis_. By the other mode, the living parent was supposed to give
rise to offspring which passed through a totally different series of
states from those exhibited by the parent, and did not return into the
cycle of the parent; this is what ought to be called _Heterogenesis_, the
offspring being altogether, and permanently, unlike the parent. The term
Heterogenesis, however, has unfortunately been used in a different sense,
and M. Milne-Edwards has therefore substituted for it _Xenogenesis_,
which means the generation of something foreign. After discussing Redi's
hypothesis of universal Biogenesis, then, I shall go on to ask how far
the growth of science justifies his other hypothesis of Xenogenesis.
The progress of the hypothesis of Biogenesis was triumphant and unchecked
for nearly a century. The application of the microscope to anatomy in the
hands of Grew, Leeuwenhoek, Swammerdam, Lyonnet, Vallisnieri, Reaurnur,
and other illustrious investigators of nature of that day, displayed such
a complexity of organisation in the lowest and minutest forms, and
everywhere revealed such a prodigality of provision for their
multiplication by germs of one sort or another, that the hypothesis of
Abiogenesis began to appear not only untrue, but absurd; and, in the
middle of the eighteenth century, when Needham and Buffon took up the
question, it was almost universally discredited.[7]
[Footnote 7: Needham, writing in 1750, says:--
"Les naturalistes modernes s'accordent unaninement a etablir, comme une
verite certaine, que toute plante vient do sa semence specifique, tout
animal d'un oeuf ou de quelque chose d'analogue preexistant dans la
plante, ou dans l'animal de meme espece qui l'a produit."--_Nouvelles
Observations_, p. 169.
"Les naturalistes out generalemente cru que les animaux microscopiques
etaient engendres par des oeufs transportes dans l'air, ou deposes dans
des eaux dormantes par des insectes volans."--_Ibid._ p. 176.]
But the skill of the microscope makers of the eighteenth century soon
reached its limit. A microscope magnifying 400 diameters was a _chef
d'oeuvre_ of the opticians of that day; and, at the same time, by no
means trustworthy. But a magnifying power of 400 diameters, even when
definition reaches the exquisite perfection of our modern achromatic
lenses, hardly suffices for the mere discernment of the smallest forms of
life. A speck, only 1/25th of an inch in diameter, has, at ten inches
from the eye, the same apparent size as an object 1/10000th of an inch in
diameter, when magnified 400 times; but forms of living matter abound,
the diameter of which is not more than 1/40000th of an inch. A filtered
infusion of hay, allowed to stand for two days, will swarm with living
things among which, any which reaches the diameter of a human red blood-
corpuscle, or about 1/3200th of an inch, is a giant. It is only by
bearing these facts in mind, that we can deal fairly with the remarkable
statements and speculations put forward by Buffon and Needham in the
middle of the eighteenth century.
When a portion of any animal or vegetable body is infused in water, it
gradually softens and disintegrates; and, as it does so, the water is
found to swarm with minute active creatures, the so-called Infusorial
Animalcules, none of which can be seen, except by the aid of the
microscope; while a large proportion belong to the category of smallest
things of which I have spoken, and which must have looked like mere dots
and lines under the ordinary microscopes of the eighteenth century.
Led by various theoretical considerations which I cannot now discuss, but
which looked promising enough in the lights of their time, Buffon and
Needham doubted the applicability of Redi's hypothesis to the infusorial
animalcules, and Needham very properly endeavoured to put the question to
an experimental test. He said to himself, If these infusorial animalcules
come from germs, their germs must exist either in the substance infused,
or in the water with which the infusion is made, or in the superjacent
air. Now the vitality of all germs is destroyed by heat. Therefore, if I
boil the infusion, cork it up carefully, cementing the cork over with
mastic, and then heat the whole vessel by heaping hot ashes over it, I
must needs kill whatever germs are present. Consequently, if Redi's
hypothesis hold good, when the infusion is taken away and allowed to
cool, no animalcules ought to be developed in it; whereas, if the
animalcules are not dependent on pre-existing germs, but are generated
from the infused substance, they ought, by and by, to make their
appearance. Needham found that, under the circumstances in which he made
his experiments, animalcules always did arise in the infusions, when a
sufficient time had elapsed to allow for their development.
In much of his work Needham was associated with Buffon, and the results
of their experiments fitted in admirably with the great French
naturalist's hypothesis of "organic molecules," according to which, life
is the indefeasible property of certain indestructible molecules of
matter, which exist in all living things, and have inherent activities by
which they are distinguished from not living matter. Each individual
living organism is formed by their temporary combination. They stand to
it in the relation of the particles of water to a cascade, or a
whirlpool; or to a mould, into which the water is poured. The form of the
organism is thus determined by the reaction between external conditions
and the inherent activities of the organic molecules of which it is
composed; and, as the stoppage of a whirlpool destroys nothing but a
form, and leaves the molecules of the water, with all their inherent
activities intact, so what we call the death and putrefaction of an
animal, or of a plant, is merely the breaking up of the form, or manner
of association, of its constituent organic molecules, which are then set
free as infusorial animalcules.
It will be perceived that this doctrine is by no means identical with
_Abiogenesis_, with which it is often confounded. On this hypothesis, a
piece of beef, or a handful of hay, is dead only in a limited sense. The
beef is dead ox, and the hay is dead grass; but the "organic molecules"
of the beef or the hay are not dead, but are ready to manifest their
vitality as soon as the bovine or herbaceous shrouds in which they are
imprisoned are rent by the macerating action of water. The hypothesis
therefore must be classified under Xenogenesis, rather than under
Abiogenesis. Such as it was, I think it will appear, to those who will be
just enough to remember that it was propounded before the birth of modern
chemistry, and of the modern optical arts, to be a most ingenious and
suggestive speculation.
But the great tragedy of Science--the slaying of a beautiful hypothesis
by an ugly fact--which is so constantly being enacted under the eyes of
philosophers, was played, almost immediately, for the benefit of Buffon
and Needham.
Once more, an Italian, the Abbe Spallanzani, a worthy successor and
representative of Redi in his acuteness, his ingenuity, and his learning,
subjected the experiments and the conclusions of Needham to a searching
criticism. It might be true that Needham's experiments yielded results
such as he had described, but did they bear out his arguments? Was it not
possible, in the first place, he had not completely excluded the air by
his corks and mastic? And was it not possible, in the second place, that
he had not sufficiently heated his infusions and the superjacent air?
Spallanzani joined issue with the English naturalist on both these pleas,
and he showed that if, in the first place, the glass vessels in which the
infusions were contained were hermetically sealed by fusing their necks,
and if, in the second place, they were exposed to the temperature of
boiling water for three-quarters of an hour,[8] no animalcules ever made
their appearance within them. It must be admitted that the experiments
and arguments of Spallanzani furnish a complete and a crushing reply to
those of Needham. But we all too often forget that it is one thing to
refute a proposition, and another to prove the truth of a doctrine which,
implicitly or explicitly, contradicts that proposition; and the advance
of science soon showed that though Needham might be quite wrong, it did
not follow that Spallanzani was quite right.
[Footnote 8: See Spallanzani, _Opere_, vi. pp. 42 and 51.]
Modern Chemistry, the birth of the latter half of the eighteenth century,
grew apace, and soon found herself face to face with the great problems
which biology had vainly tried to attack without her help. The discovery
of oxygen led to the laying of the foundations of a scientific theory of
respiration, and to an examination of the marvellous interactions of
organic substances with oxygen. The presence of free oxygen appeared to
be one of the conditions of the existence of life, and of those singular
changes in organic matters which are known as fermentation and
putrefaction. The question of the generation of the infusory animalcules
thus passed into a new phase. For what might not have happened to the
organic matter of the infusions, or to the oxygen of the air, in
Spallanzani's experiments? What security was there that the development
of life which ought to have taken place had not been checked or prevented
by these changes?
The battle had to be fought again. It was needful to repeat the
experiments under conditions which would make sure that neither the
oxygen of the air, nor the composition of the organic matter, was altered
in such a manner as to interfere with the existence of life.
Schulze and Schwann took up the question from this point of view in 1836
and 1837. The passage of air through red-hot glass tubes, or through
strong sulphuric acid, does not alter the proportion of its oxygen, while
it must needs arrest, or destroy, any organic matter which may be
contained in the air. These experimenters, therefore, contrived
arrangements by which the only air which should come into contact with a
boiled infusion should be such as had either passed through red-hot tubes
or through strong sulphuric acid. The result which they obtained was that
an infusion so treated developed no living things, while, if the same
infusion was afterwards exposed to the air, such things appeared rapidly
and abundantly. The accuracy of these experiments has been alternately
denied and affirmed. Supposing then, to be accepted, however, all that
they really proved was that the treatment to which the air was subjected
destroyed _something_ that was essential to the development of life in
the infusion. This "something" might be gaseous, fluid, or solid; that it
consisted of germs remained only an hypothesis of greater or less
probability.
Contemporaneously with these investigations a remarkable discovery was
made by Cagniard de la Tour. He found that common yeast is composed of a
vast accumulation of minute plants. The fermentation of must, or of wort,
in the fabrication of wine and of beer, is always accompanied by the
rapid growth and multiplication of these _Toruloe_. Thus, fermentation,
in so far as it was accompanied by the development of microscopical
organisms in enormous numbers, became assimilated to the decomposition of
an infusion of ordinary animal or vegetable matter; and it was an obvious
suggestion that the organisms were, in some way or other, the causes both
of fermentation and of putrefaction. The chemists, with Berzelius and
Liebig at their head, at first laughed this idea to scorn; but in 1843, a
man then very young, who has since performed the unexampled feat of
attaining to high eminence alike in Mathematics, Physics, and Physiology--
I speak of the illustrious Helmholtz--reduced the matter to the test of
experiment by a method alike elegant and conclusive. Helmholtz separated
a putrefying or a fermenting liquid from one which was simply putrescible
or fermentable by a membrane which allowed the fluids to pass through and
become intermixed, but stopped the passage of solids. The result was,
that while the putrescible or the fermentable liquids became impregnated
with the results of the putrescence or fermentation which was going on on
the other side of the membrane, they neither putrefied (in the ordinary
way) nor fermented; nor were any of the organisms which abounded in the
fermenting or putrefying liquid generated in them. Therefore the cause of
the development of these organisms must lie in something which cannot
pass through membranes; and as Helmholtz's investigations were long
antecedent to Graham's researches upon colloids, his natural conclusion
was that the agent thus intercepted must be a solid material. In point of
fact, Helmholtz's experiments narrowed the issue to this: that which
excites fermentation and putrefaction, and at the same time gives rise to
living forms in a fermentable or putrescible fluid, is not a gas and is
not a diffusible fluid; therefore it is either a colloid, or it is matter
divided into very minute solid particles.
The researches of Schroeder and Dusch in 1854, and of Schroeder alone, in
1859, cleared up this point by experiments which are simply refinements
upon those of Redi. A lump of cotton-wool is, physically speaking, a pile
of many thicknesses of a very fine gauze, the fineness of the meshes of
which depends upon the closeness of the compression of the wool. Now,
Schroeder and Dusch found, that, in the case of all the putrefiable
materials which they used (except milk and yolk of egg), an infusion
boiled, and then allowed to come into contact with no air but such as had
been filtered through cotton-wool, neither putrefied, nor fermented, nor
developed living forms. It is hard to imagine what the fine sieve formed
by the cotton-wool could have stopped except minute solid particles.
Still the evidence was incomplete until it had been positively shown,
first, that ordinary air does contain such particles; and, secondly, that
filtration through cotton-wool arrests these particles and allows only
physically pure air to pass. This demonstration has been furnished within
the last year by the remarkable experiments of Professor Tyndall. It has
been a common objection of Abiogenists that, if the doctrine of Biogeny
is true, the air must be thick with germs; and they regard this as the
height of absurdity. But nature occasionally is exceedingly unreasonable,
and Professor Tyndall has proved that this particular absurdity may
nevertheless be a reality. He has demonstrated that ordinary air is no
better than a sort of stirabout of excessively minute solid particles;
that these particles are almost wholly destructible by heat; and that
they are strained off, and the air rendered optically pure, by its being
passed through cotton-wool.
It remains yet in the order of logic, though not of history, to show that
among these solid destructible particles, there really do exist germs
capable of giving rise to the development of living forms in suitable
menstrua. This piece of work was done by M. Pasteur in those beautiful
researches which will ever render his name famous; and which, in spite of
all attacks upon them, appear to me now, as they did seven years ago,[9]
to be models of accurate experimentation and logical reasoning. He
strained air through cotton-wool, and found, as Schroeder and Dusch had
done, that it contained nothing competent to give rise to the development
of life in fluids highly fitted for that purpose. But the important
further links in the chain of evidence added by Pasteur are three. In the
first place he subjected to microscopic examination the cotton-wool which
had served as strainer, and found that sundry bodies clearly recognisable
as germs, were among the solid particles strained off. Secondly, he
proved that these germs were competent to give rise to living forms by
simply sowing them in a solution fitted for their development. And,
thirdly, he showed that the incapacity of air strained through cotton-
wool to give rise to life, was not due to any occult change effected in
the constituents of the air by the wool, by proving that the cotton-wool
might be dispensed with altogether, and perfectly free access left
between the exterior air and that in the experimental flask. If the neck
of the flask is drawn out into a tube and bent downwards; and if, after
the contained fluid has been carefully boiled, the tube is heated
sufficiently to destroy any germs which may be present in the air which
enters as the fluid cools, the apparatus may be left to itself for any
time and no life will appear in the fluid. The reason is plain. Although
there is free communication between the atmosphere laden with germs and
the germless air in the flask, contact between the two takes place only
in the tube; and as the germs cannot fall upwards, and there are no
currents, they never reach the interior of the flask. But if the tube be
broken short off where it proceeds from the flask, and free access be
thus given to germs falling vertically out of the air, the fluid, which
has remained clear and desert for months, becomes, in a few days, turbid
and full of life.
[Footnote 9: _Lectures to Working Men on the Causes of the Phenomena of
Organic Nature_, 1863. (See Vol. II. of these Essays.)]
These experiments have been repeated over and over again by independent
observers with entire success; and there is one very simple mode of
seeing the facts for one's self, which I may as well describe.
Prepare a solution (much used by M. Pasteur, and often called "Pasteur's
solution") composed of water with tartrate of ammonia, sugar, and yeast-
ash dissolved therein.[10] Divide it into three portions in as many
flasks; boil all three for a quarter of an hour; and, while the steam is
passing out, stop the neck of one with a large plug of cotton-wool, so
that this also may be thoroughly steamed. Now set the flasks aside to
cool, and, when their contents are cold, add to one of the open ones a
drop of filtered infusion of hay which has stood for twenty-four hours,
and is consequently hill of the active and excessively minute organisms
known as _Bacteria_. In a couple of days of ordinary warm weather the
contents of this flask will be milky from the enormous multiplication of
_Bacteria_. The other flask, open and exposed to the air, will, sooner or
later, become milky with _Bacteria_, and patches of mould may appear in
it; while the liquid in the flask, the neck of which is plugged with
cotton-wool, will remain clear for an indefinite time. I have sought in
vain for any explanation of these facts, except the obvious one, that the
air contains germs competent to give rise to _Bacteria_, such as those
with which the first solution has been knowingly and purposely
inoculated, and to the mould-_Fungi_. And I have not yet been able to
meet with any advocate of Abiogenesis who seriously maintains that the
atoms of sugar, tartrate of ammonia, yeast-ash, and water, under no
influence but that of free access of air and the ordinary temperature,
re-arrange themselves and give rise to the protoplasm of _Bacterium_. But
the alternative is to admit that these _Bacteria_ arise from germs in the
air; and if they are thus propagated, the burden of proof that other like
forms are generated in a different manner, must rest with the assertor of
that proposition.
[Footnote 10: Infusion of hay treated in the same way yields similar
results; but as it contains organic matter, the argument which follows
cannot be based upon it.]
To sum up the effect of this long chain of evidence:--
It is demonstrable that a fluid eminently fit for the development of the
lowest forms of life, but which contains neither germs, nor any protein
compound, gives rise to living things in great abundance if it is exposed
to ordinary air; while no such development takes place, if the air with
which it is in contact is mechanically freed from the solid particles
which ordinarily float in it, and which may be made visible by
appropriate means.
It is demonstrable that the great majority of these particles are
destructible by heat, and that some of them are germs, or living
particles, capable of giving rise to the same forms of life as those
which appear when the fluid is exposed to unpurified air.
It is demonstrable that inoculation of the experimental fluid with a drop
of liquid known to contain living particles gives rise to the same
phenomena as exposure to unpurified air.
And it is further certain that these living particles are so minute that
the assumption of their suspension in ordinary air presents not the
slightest difficulty. On the contrary, considering their lightness and
the wide diffusion of the organisms which produce them, it is impossible
to conceive that they should not be suspended in the atmosphere in
myriads.
Thus the evidence, direct and indirect, in favour of _Biogenesis_ for all
known forms of life must, I think, be admitted to be of great weight.
On the other side, the sole assertions worthy of attention are that
hermetically sealed fluids, which have been exposed to great and long-
continued heat, have sometimes exhibited living forms of low organisation
when they have been opened.
The first reply that suggests itself is the probability that there must
be some error about these experiments, because they are performed on an
enormous scale every day with quite contrary results. Meat, fruits,
vegetables, the very materials of the most fermentable and putrescible
infusions, are preserved to the extent, I suppose I may say, of thousands
of tons every year, by a method which is a mere application of
Spallanzani's experiment. The matters to be preserved are well boiled in
a tin case provided with a small hole, and this hole is soldered up when
all the air in the case has been replaced by steam. By this method they
may be kept for years without putrefying, fermenting, or getting mouldy.
Now this is not because oxygen is excluded, inasmuch as it is now proved
that free oxygen is not necessary for either fermentation or
putrefaction. It is not because the tins are exhausted of air, for
_Vibriones_ and _Bacteria_ live, as Pasteur has shown, without air or
free oxygen. It is not because the boiled meats or vegetables are not
putrescible or fermentable, as those who have had the misfortune to be in
a ship supplied with unskilfully closed tins well know. What is it,
therefore, but the exclusion of germs? I think that Abiogenists are bound
to answer this question before they ask us to consider new experiments of
precisely the same order.
And in the next place, if the results of the experiments I refer to are
really trustworthy, it by no means follows that Abiogenesis has taken
place. The resistance of living matter to heat is known to vary within
considerable limits, and to depend, to some extent, upon the chemical and
physical qualities of the surrounding medium. But if, in the present
state of science, the alternative is offered us,--either germs can stand
a greater heat than has been supposed, or the molecules of dead matter,
for no valid or intelligible reason that is assigned, are able to re-
arrange themselves into living bodies, exactly such as can be
demonstrated to be frequently produced in another way,--I cannot
understand how choice can be, even for a moment, doubtful.
But though I cannot express this conviction of mine too strongly, I must
carefully guard myself against the supposition that I intend to suggest
that no such thing as Abiogenesis ever has taken place in the past, or
ever will take place in the future. With organic chemistry, molecular
physics, and physiology yet in their infancy, and every day making
prodigious strides, I think it would be the height of presumption for any
man to say that the conditions under which matter assumes the properties
we call "vital" may not, some day, be artificially brought together. All
I feel justified in affirming is, that I see no reason for believing that
the feat has been performed yet.
And looking back through the prodigious vista of the past, I find no
record of the commencement of life, and therefore I am devoid of any
means of forming a definite conclusion as to the conditions of its
appearance. Belief, in the scientific sense of the word, is a serious
matter, and needs strong foundations. To say, therefore, in the admitted
absence of evidence, that I have any belief as to the mode in which the
existing forms of life have originated, would be using words in a wrong
sense. But expectation is permissible where belief is not; and if it were
given me to look beyond the abyss of geologically recorded time to the
still more remote period when the earth was passing through physical and
chemical conditions, which it can no more see again than a man can recall
his infancy, I should expect to be a witness of the evolution of living
protoplasm from not living matter. I should expect to see it appear under
forms of great simplicity, endowed, like existing fungi, with the power
of determining the formation of new protoplasm from such matters as
ammonium carbonates, oxalates and tartrates, alkaline and earthy
phosphates, and water, without the aid of light. That is the expectation
to which analogical reasoning leads me; but I beg you once more to
recollect that I have no right to call my opinion anything but an act of
philosophical faith.
So much for the history of the progress of Redi's great doctrine of
Biogenesis, which appears to me, with the limitations I have expressed,
to be victorious along the whole line at the present day.
As regards the second problem offered to us by Redi, whether Xenogenesis
obtains, side by side with Homogenesis,--whether, that is, there exist
not only the ordinary living things, giving rise to offspring which run
through the same cycle as themselves, but also others, producing
offspring which are of a totally different character from themselves,--
the researches of two centuries have led to a different result. That the
grubs found in galls are no product of the plants on which the galls
grow, but are the result of the introduction of the eggs of insects into
the substance of these plants, was made out by Vallisnieri, Reaumur, and
others, before the end of the first half of the eighteenth century. The
tapeworms, bladderworms, and flukes continued to be a stronghold of the
advocates of Xenogenesis for a much longer period. Indeed, it is only
within the last thirty years that the splendid patience of Von Siebold,
Van Beneden, Leuckart, Kuechenmeister, and other helminthologists, has
succeeded in tracing every such parasite, often through the strangest
wanderings and metamorphoses, to an egg derived from a parent, actually
or potentially like itself; and the tendency of inquiries elsewhere has
all been in the same direction. A plant may throw off bulbs, but these,
sooner or later, give rise to seeds or spores, which develop into the
original form. A polype may give rise to Medusae, or a pluteus to an
Echinoderm, but the Medusa and the Echinoderm give rise to eggs which
produce polypes or glutei, and they are therefore only stages in the
cycle of life of the species.
But if we turn to pathology, it offers us some remarkable approximations
to true Xenogenesis.
As I have already mentioned, it has been known since the time of
Vallisnieri and of Reaumur, that galls in plants, and tumours in cattle,
are caused by insects, which lay their eggs in those parts of the animal
or vegetable frame of which these morbid structures are outgrowths.
Again, it is a matter of familiar experience to everybody that mere
pressure on the skin will give rise to a corn. Now the gall, the tumour,
and the corn are parts of the living body, which have become, to a
certain degree, independent and distinct organisms. Under the influence
of certain external conditions, elements of the body, which should have
developed in due subordination to its general plan, set up for themselves
and apply the nourishment which they receive to their own purposes.
From such innocent productions as corns and warts, there are all
gradations to the serious tumours which, by their mere size and the
mechanical obstruction they cause, destroy the organism out of which they
are developed; while, finally, in those terrible structures known as
cancers, the abnormal growth has acquired powers of reproduction and
multiplication, and is only morphologically distinguishable from the
parasitic worm, the life of which is neither more nor less closely bound
up with that of the infested organism.
If there were a kind of diseased structure, the histological elements of
which were capable of maintaining a separate and independent existence
out of the body, it seems to me that the shadowy boundary between morbid
growth and Xenogenesis would be effaced. And I am inclined to think that
the progress of discovery has almost brought us to this point already. I
have been favoured by Mr. Simon with an early copy of the last published
of the valuable "Reports on the Public Health," which, in his capacity of
their medical officer, he annually presents to the Lords of the Privy
Council. The appendix to this report contains an introductory essay "On
the Intimate Pathology of Contagion," by Dr. Burdon-Sanderson, which is
one of the clearest, most comprehensive, and well-reasoned discussions of
a great question which has come under my notice for a long time. I refer
you to it for details and for the authorities for the statements I am
about to make.
You are familiar with what happens in vaccination. A minute cut is made
in the skin, and an infinitesimal quantity of vaccine matter is inserted
into the wound. Within a certain time a vesicle appears in the place of
the wound, and the fluid which distends this vesicle is vaccine matter,
in quantity a hundred or a thousandfold that which was originally
inserted. Now what has taken place in the course of this operation? Has
the vaccine matter, by its irritative property, produced a mere blister,
the fluid of which has the same irritative property? Or does the vaccine
matter contain living particles, which have grown and multiplied where
they have been planted? The observations of M. Chauveau, extended and
confirmed by Dr. Sanderson himself, appear to leave no doubt upon this
head. Experiments, similar in principle to those of Helmholtz on
fermentation and putrefaction, have proved that the active element in the
vaccine lymph is non-diffusible, and consists of minute particles not
exceeding 1/20000th of an inch in diameter, which are made visible in the
lymph by the microscope. Similar experiments have proved that two of the
most destructive of epizootic diseases, sheep-pox and glanders, are also
dependent for their existence and their propagation upon extremely small
living solid particles, to which the title of _microzymes_ is applied. An
animal suffering under either of these terrible diseases is a source of
infection and contagion to others, for precisely the same reason as a tub
of fermenting beer is capable of propagating its fermentation by
"infection," or "contagion," to fresh wort. In both cases it is the solid
living particles which are efficient; the liquid in which they float, and
at the expense of which they live, being altogether passive.
Now arises the question, are these microzymes the results of
_Homogenesis_, or of _Xenogenesis?_ are they capable, like the
_Toruloe_ of yeast, of arising only by the development of pre-existing
germs? or may they be, like the constituents of a nut-gall, the results
of a modification and individualisation of the tissues of the body in
which they are found, resulting from the operation of certain conditions?
Are they parasites in the zoological sense, or are they merely what
Virchow has called "heterologous growths"? It is obvious that this
question has the most profound importance, whether we look at it from a
practical or from a theoretical point of view. A parasite may be stamped
out by destroying its germs, but a pathological product can only be
annihilated by removing the conditions which give rise to it.
It appears to me that this great problem will have to be solved for each
zymotic disease separately, for analogy cuts two ways. I have dwelt upon
the analogy of pathological modification, which is in favour of the
xenogenetic origin of microzymes; but I must now speak of the equally
strong analogies in favour of the origin of such pestiferous particles by
the ordinary process of the generation of like from like.
It is, at present, a well-established fact that certain diseases, both of
plants and of animals, which have all the characters of contagious and
infectious epidemics, are caused by minute organisms. The smut of wheat
is a well-known instance of such a disease, and it cannot be doubted that
the grape-disease and the potato-disease fall under the same category.
Among animals, insects are wonderfully liable to the ravages of
contagious and infectious diseases caused by microscopic _Fungi_.
In autumn, it is not uncommon to see flies motionless upon a window-pane,
with a sort of magic circle, in white, drawn round them. On microscopic
examination, the magic circle is found to consist of innumerable spores,
which have been thrown off in all directions by a minute fungus called
_Empusa muscoe_, the spore-forming filaments of which stand out like a
pile of velvet from the body of the fly. These spore-forming filaments
are connected with others which fill the interior of the fly's body like
so much fine wool, having eaten away and destroyed the creature's
viscera. This is the full-grown condition of the _Empusa_. If traced back
to its earliest stages, in flies which are still active, and to all
appearance healthy, it is found to exist in the form of minute corpuscles
which float in the blood of the fly. These multiply and lengthen into
filaments, at the expense of the fly's substance; and when they have at
last killed the patient, they grow out of its body and give off spores.
Healthy flies shut up with diseased ones catch this mortal disease, and
perish like the others. A most competent observer, M. Cohn, who studied
the development of the _Empusa_ very carefully, was utterly unable to
discover in what manner the smallest germs of the _Empusa_ got into the
fly. The spores could not be made to give rise to such germs by
cultivation; nor were such germs discoverable in the air, or in the food
of the fly. It looked exceedingly like a case of Abiogenesis, or, at any
rate, of Xenogenesis; and it is only quite recently that the real course
of events has been made out. It has been ascertained, that when one of
the spores falls upon the body of a fly, it begins to germinate, and
sends out a process which bores its way through the fly's skin; this,
having reached the interior cavities of its body, gives off the minute
floating corpuscles which are the earliest stage of the _Empusa_. The
disease is "contagious," because a healthy fly coming in contact with a
diseased one, from which the spore-bearing filaments protrude, is pretty
sure to carry off a spore or two. It is "infectious" because the spores
become scattered about all sorts of matter in the neighbourhood of the
slain flies.
The silkworm has long been known to be subject to a very fatal and
infectious disease called the _Muscardine_. Audouin transmitted it by
inoculation. This disease is entirely due to the development of a fungus,
_Botrytis Bassiana_, in the body of the caterpillar; and its
contagiousness and infectiousness are accounted for in the same way as
those of the fly-disease. But, of late years, a still more serious
epizootic has appeared among the silkworms; and I may mention a few facts
which will give you some conception of the gravity of the injury which it
has inflicted on France alone.
The production of silk has been for centuries an important branch of
industry in Southern France, and in the year 1853 it had attained such a
magnitude that the annual produce of the French sericulture was estimated
to amount to a tenth of that of the whole world, and represented a money-
value of 117,000,000 francs, or nearly five millions sterling. What may
be the sum which would represent the money-value of all the industries
connected with the working up of the raw silk thus produced, is more than
I can pretend to estimate. Suffice it to say, that the city of Lyons is
built upon French silk as much as Manchester was upon American cotton
before the civil war.
Silkworms are liable to many diseases; and, even before 1853, a peculiar
epizootic, frequently accompanied by the appearance of dark spots upon
the skin (whence the name of "Pebrine" which it has received), had been
noted for its mortality. But in the years following 1853 this malady
broke out with such extreme violence, that, in 1858, the silk-crop was
reduced to a third of the amount which it had reached in 1853; and, up
till within the last year or two, it has never attained half the yield of
1853. This means not only that the great number of people engaged in silk
growing are some thirty millions sterling poorer than they might have
been; it means not only that high prices have had to be paid for imported
silkworm eggs, and that, after investing his money in them, in paying for
mulberry-leaves and for attendance, the cultivator has constantly seen
his silkworms perish and himself plunged in ruin; but it means that the
looms of Lyons have lacked employment, and that, for years, enforced
idleness and misery have been the portion of a vast population which, in
former days, was industrious and well-to-do.
In 1858 the gravity of the situation caused the French Academy of
Sciences to appoint Commissioners, of whom a distinguished naturalist, M.
de Quatrefages, was one, to inquire into the nature of this disease, and,
if possible, to devise some means of staying the plague. In reading the
Report[11] made by M. de Quatrefages in 1859, it is exceedingly
interesting to observe that his elaborate study of the Pebrine forced the
conviction upon his mind that, in its mode of occurrence and propagation,
the disease of the silkworm is, in every respect, comparable to the
cholera among mankind. But it differs from the cholera, and so far is a
more formidable malady, in being hereditary, and in being, under some
circumstances, contagious as well as infectious.
[Footnote 11: _Etudes sur les Maladies actuelles des Vers a Soie_, p.
53.]
The Italian naturalist, Filippi, discovered in the blood of the silkworms
affected by this strange disorder a multitude of cylindrical corpuscles,
each about 1/6000th of an inch long. These have been carefully studied by
Lebert, and named by him _Panhistophyton_; for the reason that in
subjects in which the disease is strongly developed, the corpuscles swarm
in every tissue and organ of the body, and even pass into the undeveloped
eggs of the female moth. But are these corpuscles causes, or mere
concomitants, of the disease? Some naturalists took one view and some
another; and it was not until the French Government, alarmed by the
continued ravages of the malady, and the inefficiency of the remedies
which had been suggested, despatched M. Pasteur to study it, that the
question received its final settlement; at a great sacrifice, not only of
the time and peace of mind of that eminent philosopher, but, I regret to
have to add, of his health.
But the sacrifice has not been in vain. It is now certain that this
devastating, cholera-like, Pebrine, is the effect of the growth and
multiplication of the _Panhistophyton_ in the silkworm. It is contagious
and infectious, because the corpuscles of the _Panhistophyton_ pass away
from the bodies of the diseased caterpillars, directly or indirectly, to
the alimentary canal of healthy silkworms in their neighbourhood; it is
hereditary because the corpuscles enter into the eggs while they are
being formed, and consequently are carried within them when they are
laid; and for this reason, also, it presents the very singular
peculiarity of being inherited only on the mother's side. There is not a
single one of all the apparently capricious and unaccountable phenomena
presented by the Pebrine, but has received its explanation from the fact
that the disease is the result of the presence of the microscopic
organism, _Panhistophyton_.
Such being the facts with respect to the Pebrine, what are the
indications as to the method of preventing it? It is obvious that this
depends upon the way in which the _Panhistophyton_ is generated. If it
may be generated by Abiogenesis, or by Xenogenesis, within the silkworm
or its moth, the extirpation of the disease must depend upon the
prevention of the occurrence of the conditions under which this
generation takes place. But if, on the other hand, the _Panhistophyton_
is an independent organism, which is no more generated by the silkworm
than the mistletoe is generated by the apple-tree or the oak on which it
grows, though it may need the silkworm for its development in the same
way as the mistletoe needs the tree, then the indications are totally
different. The sole thing to be done is to get rid of and keep away the
germs of the _Panhistophyton_. As might be imagined, from the course of
his previous investigations, M. Pasteur was led to believe that the
latter was the right theory; and, guided by that theory, he has devised a
method of extirpating the disease, which has proved to be completely
successful wherever it has been properly carried out.
There can be no reason, then, for doubting that, among insects,
contagious and infectious diseases, of great malignity, are caused by
minute organisms which are produced from pre-existing germs, or by
homogenesis; and there is no reason, that I know of, for believing that
what happens in insects may not take place in the highest animals.
Indeed, there is already strong evidence that some diseases of an
extremely malignant and fatal character to which man is subject, are as
much the work of minute organisms as is the Pebrine. I refer for this
evidence to the very striking facts adduced by Professor Lister in his
various well-known publications on the antiseptic method of treatment. It
appears to me impossible to rise from the perusal of those publications
without a strong conviction that the lamentable mortality which so
frequently dogs the footsteps of the most skilful operator, and those
deadly consequences of wounds and injuries which seem to haunt the very
walls of great hospitals, and are, even now, destroying more men than die
of bullet or bayonet, are due to the importation of minute organisms into
wounds, and their increase and multiplication; and that the surgeon who
saves most lives will be he who best works out the practical consequences
of the hypothesis of Redi.
I commenced this Address by asking you to follow me in an attempt to
trace the path which has been followed by a scientific idea, in its long
and slow progress from the position of a probable hypothesis to that of
an established law of nature. Our survey has not taken us into very
attractive regions; it has lain, chiefly, in a land flowing with the
abominable, and peopled with mere grubs and mouldiness. And it may be
imagined with what smiles and shrugs, practical and serious
contemporaries of Redi and of Spallanzani may have commented on the waste
of their high abilities in toiling at the solution of problems which,
though curious enough in themselves, could be of no conceivable utility
to mankind.
Nevertheless, you will have observed that before we had travelled very
far upon our road, there appeared, on the right hand and on the left,
fields laden with a harvest of golden grain, immediately convertible into
those things which the most solidly practical men will admit to have
value--viz., money and life.
The direct loss to France caused by the Pebrine in seventeen years cannot
be estimated at less than fifty millions sterling; and if we add to this
what Redi's idea, in Pasteur's hands, has done for the wine-grower and
for the vinegar-maker, and try to capitalise its value, we shall find
that it will go a long way towards repairing the money losses caused by
the frightful and calamitous war of this autumn. And as to the equivalent
of Redi's thought in life, how can we over-estimate the value of that
knowledge of the nature of epidemic and epizootic diseases, and
consequently of the means of checking, or eradicating them, the dawn of
which has assuredly commenced?
Looking back no further than ten years, it is possible to select three
(1863, 1864, and 1869) in which the total number of deaths from scarlet-
fever alone amounted to ninety thousand. That is the return of killed,
the maimed and disabled being left out of sight. Why, it is to be hoped
that the list of killed in the present bloodiest of all wars will not
amount to more than this! But the facts which I have placed before you
must leave the least sanguine without a doubt that the nature and the
causes of this scourge will, one day, be as well understood as those of
the Pebrine are now; and that the long-suffered massacre of our innocents
will come to an end.
And thus mankind will have one more admonition that "the people perish
for lack of knowledge"; and that the alleviation of the miseries, and the
promotion of the welfare, of men must be sought, by those who will not
lose their pains, in that diligent, patient, loving study of all the
multitudinous aspects of Nature, the results of which constitute exact
knowledge, or Science. It is the justification and the glory of this
great meeting that it is gathered together for no other object than the
advancement of the moiety of science which deals with those phenomena of
nature which we call physical. May its endeavours be crowned with a full
measure of success!
IX
GEOLOGICAL CONTEMPORANEITY AND PERSISTENT TYPES OF LIFE
[1862]
Merchants occasionally go through a wholesome, though troublesome and not
always satisfactory, process which they term "taking stock." After all
the excitement of speculation, the pleasure of gain, and the pain of
loss, the trader makes up his mind to face facts and to learn the exact
quantity and quality of his solid and reliable possessions.
The man of science does well sometimes to imitate this procedure; and,
forgetting for the time the importance of his own small winnings, to re-
examine the common stock in trade, so that he may make sure how far the
stock of bullion in the cellar--on the faith of whose existence so much
paper has been circulating--is really the solid gold of truth.
The Anniversary Meeting of the Geological Society seems to be an occasion
well suited for an undertaking of this kind--for an inquiry, in fact,
into the nature and value of the present results of palaeontological
investigation; and the more so, as all those who have paid close
attention to the late multitudinous discussions in which palaeontology is
implicated, must have felt the urgent necessity of some such scrutiny.
First in order, as the most definite and unquestionable of all the
results of palaeontology, must be mentioned the immense extension and
impulse given to botany, zoology, and comparative anatomy, by the
investigation of fossil remains. Indeed, the mass of biological facts has
been so greatly increased, and the range of biological speculation has
been so vastly widened, by the researches of the geologist and
palaeontologist, that it is to be feared there are naturalists in
existence who look upon geology as Brindley regarded rivers. "Rivers,"
said the great engineer, "were made to feed canals;" and geology, some
seem to think, was solely created to advance comparative anatomy.
Were such a thought justifiable, it could hardly expect to be received
with favour by this assembly. But it is not justifiable. Your favourite
science has her own great aims independent of all others; and if,
notwithstanding her steady devotion to her own progress, she can scatter
such rich alms among her sisters, it should be remembered that her
charity is of the sort that does not impoverish, but "blesseth him that
gives and him that takes."
Regard the matter as we will, however, the facts remain. Nearly 40,000
species of animals and plants have been added to the Systema Naturae by
palaeontological research. This is a living population equivalent to that
of a new continent in mere number; equivalent to that of a new
hemisphere, if we take into account the small population of insects as
yet found fossil, and the large proportion and peculiar organisation of
many of the Vertebrata.
But, beyond this, it is perhaps not too much to say that, except for the
necessity of interpreting palaeontological facts, the laws of distribution
would have received less careful study; while few comparative anatomists
(and those not of the first order) would have been induced by mere love
of detail, as such, to study the minutiae of osteology, were it not that
in such minutiae lie the only keys to the most interesting riddles offered
by the extinct animal world.
These assuredly are great and solid gains. Surely it is matter for no
small congratulation that in half a century (for palaeontology, though it
dawned earlier, came into full day only with Cuvier) a subordinate branch
of biology should have doubled the value and the interest of the whole
group of sciences to which it belongs.
But this is not all. Allied with geology, palaeontology has established
two laws of inestimable importance: the first, that one and the same area
of the earth's surface has been successively occupied by very different
kinds of living beings; the second, that the order of succession
established in one locality holds good, approximately, in all.
The first of these laws is universal and irreversible; the second is an
induction from a vast number of observations, though it may possibly, and
even probably, have to admit of exceptions. As a consequence of the
second law, it follows that a peculiar relation frequently subsists
between series of strata containing organic remains, in different
localities. The series resemble one another not only in virtue of a
general resemblance of the organic remains in the two, but also in virtue
of a resemblance in the order and character of the serial succession in
each. There is a resemblance of arrangement; so that the separate terms
of each series, as well as the whole series, exhibit a correspondence.
Succession implies time; the lower members of an undisturbed series of
sedimentary rocks are certainly older than the upper; and when the notion
of age was once introduced as the equivalent of succession, it was no
wonder that correspondence in succession came to be looked upon as a
correspondence in age, or "contemporaneity." And, indeed, so long as
relative age only is spoken of, correspondence in succession _is_
correspondence in age; it is _relative_ contemporaneity.
But it would have been very much better for geology if so loose and
ambiguous a word as "contemporaneous" had been excluded from her
terminology, and if, in its stead, some term expressing similarity of
serial relation, and excluding the notion of time altogether, had been
employed to denote correspondence in position in two or more series of
strata.
In anatomy, where such correspondence of position has constantly to be
spoken of, it is denoted by the word "homology" and its derivatives; and
for Geology (which after all is only the anatomy and physiology of the
earth) it might be well to invent some single word, such as "homotaxis"
(similarity of order), in order to express an essentially similar idea.
This, however, has not been done, and most probably the inquiry will at
once be made--To what end burden science with a new and strange term in
place of one old, familiar, and part of our common language?
The reply to this question will become obvious as the inquiry into the
results of palaeontology is pushed further.
Those whose business it is to acquaint themselves specially with the
works of palaeontologists, in fact, will be fully aware that very few, if
any, would rest satisfied with such a statement of the conclusions of
their branch of biology as that which has just been given.
Our standard repertories of palaeontology profess to teach us far higher
things--to disclose the entire succession of living forms upon the
surface of the globe; to tell us of a wholly different distribution of
climatic conditions in ancient times; to reveal the character of the
first of all living existences; and to trace out the law of progress from
them to us.
It may not be unprofitable to bestow on these professions a somewhat more
critical examination than they have hitherto received, in order to
ascertain how far they rest on an irrefragable basis; or whether, after
all, it might not be well for palaeontologists to learn a little more
carefully that scientific "ars artium," the art of saying "I don't know."
And to this end let us define somewhat more exactly the extent of these
pretensions of palaeontology.
Every one is aware that Professor Bronn's "Untersuchungen" and Professor
Pictet's "Traite de Paleontologie" are works of standard authority,
familiarly consulted by every working palaeontologist. It is desirable to
speak of these excellent books, and of their distinguished authors, with
the utmost respect, and in a tone as far as possible removed from carping
criticism; indeed, if they are specially cited in this place, it is
merely in justification of the assertion that the following propositions,
which may be found implicitly, or explicitly, in the works in question,
are regarded by the mass of palaeontologists and geologists, not only on
the Continent but in this country, as expressing some of the best-
established results of palaeontology. Thus:--
Animals and plants began their existence together, not long after the
commencement of the deposition of the sedimentary rocks; and then
succeeded one another, in such a manner, that totally distinct faunae and
florae occupied the whole surface of the earth, one after the other, and
during distinct epochs of time.
A geological formation is the sum of all the strata deposited over the
whole surface of the earth during one of these epochs: a geological fauna
or flora is the sum of all the species of animals or plants which
occupied the whole surface of the globe, during one of these epochs.
The population of the earth's surface was at first very similar in all
parts, and only from the middle of the Tertiary epoch onwards, began to
show a distinct distribution in zones.
The constitution of the original population, as well as the numerical
proportions of its members, indicates a warmer and, on the whole,
somewhat tropical climate, which remained tolerably equable throughout
the year. The subsequent distribution of living beings in zones is the
result of a gradual lowering of the general temperature, which first
began to be felt at the poles.
It is not now proposed to inquire whether these doctrines are true or
false; but to direct your attention to a much simpler though very
essential preliminary question--What is their logical basis? what are the
fundamental assumptions upon which they all logically depend? and what is
the evidence on which those fundamental propositions demand our assent?
These assumptions are two: the first, that the commencement of the
geological record is coeval with the commencement of life on the globe;
the second, that geological contemporaneity is the same thing as
chronological synchrony. Without the first of these assumptions there
would of course be no ground for any statement respecting the
commencement of life; without the second, all the other statements cited,
every one of which implies a knowledge of the state of different parts of
the earth at one and the same time, will be no less devoid of
demonstration.
The first assumption obviously rests entirely on negative evidence. This
is, of course, the only evidence that ever can be available to prove the
commencement of any series of phenomena; but, at the same time, it must
be recollected that the value of negative evidence depends entirely on
the amount of positive corroboration it receives. If A.B. wishes to prove
an _alibi_, it is of no use for him to get a thousand witnesses simply to
swear that they did not see him in such and such a place, unless the
witnesses are prepared to prove that they must have seen him had he been
there. But the evidence that animal life commenced with the Lingula-
flags, _e.g._, would seem to be exactly of this unsatisfactory
uncorroborated sort. The Cambrian witnesses simply swear they "haven't
seen anybody their way"; upon which the counsel for the other side
immediately puts in ten or twelve thousand feet of Devonian sandstones to
make oath they never saw a fish or a mollusk, though all the world knows
there were plenty in their time.
But then it is urged that, though the Devonian rocks in one part of the
world exhibit no fossils, in another they do, while the lower Cambrian
rocks nowhere exhibit fossils, and hence no living being could have
existed in their epoch.
To this there are two replies: the first that the observational basis of
the assertion that the lowest rocks are nowhere fossiliferous is an
amazingly small one, seeing how very small an area, in comparison to that
of the whole world, has yet been fully searched; the second, that the
argument is good for nothing unless the unfossiliferous rocks in question
were not only _contemporaneous_ in the geological sense, but
_synchronous_ in the chronological sense. To use the _alibi_ illustration
again. If a man wishes to prove he was in neither of two places, A and B,
on a given day, his witnesses for each place must be prepared to answer
for the whole day. If they can only prove that he was not at A in the
morning, and not at B in the afternoon, the evidence of his absence from
both is nil, because he might have been at B in the morning and at A in
the afternoon.
Thus everything depends upon the validity of the second assumption. And
we must proceed to inquire what is the real meaning of the word
"contemporaneous" as employed by geologists. To this end a concrete
example may be taken.
The Lias of England and the Lias of Germany, the Cretaceous rocks of
Britain and the Cretaceous rocks of Southern India, are termed by
geologists "contemporaneous" formations; but whenever any thoughtful
geologist is asked whether he means to say that they were deposited
synchronously, he says, "No,--only within the same great epoch." And if,
in pursuing the inquiry, he is asked what may be the approximate value in
time of a "great epoch"--whether it means a hundred years, or a thousand,
or a million, or ten million years--his reply is, "I cannot tell."
If the further question be put, whether physical geology is in possession
of any method by which the actual synchrony (or the reverse) of any two
distant deposits can be ascertained, no such method can be heard of; it
being admitted by all the best authorities that neither similarity of
mineral composition, nor of physical character, nor even direct
continuity of stratum, are _absolute_ proofs of the synchronism of even
approximated sedimentary strata: while, for distant deposits, there seems
to be no kind of physical evidence attainable of a nature competent to
decide whether such deposits were formed simultaneously, or whether they
possess any given difference of antiquity. To return to an example
already given: All competent authorities will probably assent to the
proposition that physical geology does not enable us in any way to reply
to this question--Were the British Cretaceous rocks deposited at the same
time as those of India, or are they a million of years younger or a
million of years older?
Is palaeontology able to succeed where physical geology fails? Standard
writers on palaeontology, as has been seen, assume that she can. They take
it for granted, that deposits containing similar organic remains are
synchronous--at any rate in a broad sense; and yet, those who will study
the eleventh and twelfth chapters of Sir Henry De La Beche's remarkable
"Researches in Theoretical Geology," published now nearly thirty years
ago, and will carry out the arguments there most luminously stated, to
their logical consequences, may very easily convince themselves that even
absolute identity of organic contents is no proof of the synchrony of
deposits, while absolute diversity is no proof of difference of date. Sir
Henry De La Beche goes even further, and adduces conclusive evidence to
show that the different parts of one and the same stratum, having a
similar composition throughout, containing the same organic remains, and
having similar beds above and below it, may yet differ to any conceivable
extent in age.
Edward Forbes was in the habit of asserting that the similarity of the
organic contents of distant formations was _prima facie_ evidence, not of
their similarity, but of their difference of age; and holding as he did
the doctrine of single specific centres, the conclusion was as legitimate
as any other; for the two districts must have been occupied by migration
from one of the two, or from an intermediate spot, and the chances
against exact coincidence of migration and of imbedding are infinite.
In point of fact, however, whether the hypothesis of single or of
multiple specific centres be adopted, similarity of organic contents
cannot possibly afford any proof of the synchrony of the deposits which
contain them; on the contrary, it is demonstrably compatible with the
lapse of the most prodigious intervals of time, and with the
interposition of vast changes in the organic and inorganic worlds,
between the epochs in which such deposits were formed.
On what amount of similarity of their faunae is the doctrine of the
contemporaneity of the European and of the North American Silurians
based? In the last edition of Sir Charles Lyell's "Elementary Geology" it
is stated, on the authority of a former President of this Society, the
late Daniel Sharpe, that between 30 and 40 per cent. of the species of
Silurian Mollusca are common to both sides of the Atlantic. By way of due
allowance for further discovery, let us double the lesser number and
suppose that 60 per cent. of the species are common to the North American
and the British Silurians. Sixty per cent. of species in common is, then,
proof of contemporaneity.
Now suppose that, a million or two of years hence, when Britain has made
another dip beneath the sea and has come up again, some geologist applies
this doctrine, in comparing the strata laid bare by the upheaval of the
bottom, say, of St. George's Channel with what may then remain of the
Suffolk Crag. Reasoning in the same way, he will at once decide the
Suffolk Crag and the St. George's Channel beds to be contemporaneous;
although we happen to know that a vast period (even in the geological
sense) of time, and physical changes of almost unprecedented extent,
separate the two. But if it be a demonstrable fact that strata
containing more than 60 or 70 per cent. of species of Mollusca in common,
and comparatively close together, may yet be separated by an amount of
geological time sufficient to allow of some of the greatest physical
changes the world has seen, what becomes of that sort of contemporaneity
the sole evidence of which is a similarity of facies, or the identity of
half a dozen species, or of a good many genera?
And yet there is no better evidence for the contemporaneity assumed by
all who adopt the hypothesis of universal faunae and florae, of a
universally uniform climate, and of a sensible cooling of the globe
during geological time.
There seems, then, no escape from the admission that neither physical
geology, nor palaeontology, possesses any method by which the absolute
synchronism of two strata can be demonstrated. All that geology can prove
is local order of succession. It is mathematically certain that, in any
given vertical linear section of an undisturbed series of sedimentary
deposits, the bed which lies lowest is the oldest. In many other vertical
linear sections of the same series, of course, corresponding beds will
occur in a similar order; but, however great may be the probability, no
man can say with absolute certainty that the beds in the two sections
were synchronously deposited. For areas of moderate extent, it is
doubtless true that no practical evil is likely to result from assuming
the corresponding beds to be synchronous or strictly contemporaneous; and
there are multitudes of accessory circumstances which may fully justify
the assumption of such synchrony. But the moment the geologist has to
deal with large areas, or with completely separated deposits, the
mischief of confounding that "homotaxis" or "similarity of arrangement,"
which _can_ be demonstrated, with "synchrony" or "identity of date," for
which there is not a shadow of proof, under the one common term of
"contemporaneity" becomes incalculable, and proves the constant source of
gratuitous speculations.
For anything that geology or palaeontology are able to show to the
contrary, a Devonian fauna and flora in the British Islands may have been
contemporaneous with Silurian life in North America, and with a
Carboniferous fauna and flora in Africa. Geographical provinces and zones
may have been as distinctly marked in the Palaeozoic epoch as at present,
and those seemingly sudden appearances of new genera and species, which
we ascribe to new creation, may be simple results of migration.
It may be so; it may be otherwise. In the present condition of our
knowledge and of our methods, one verdict--"not proven, and not
provable"--must be recorded against all the grand hypotheses of the
palaeontologist respecting the general succession of life on the globe.
The order and nature of terrestrial life, as a whole, are open questions.
Geology at present provides us with most valuable topographical records,
but she has not the means of working them into a universal history. Is
such a universal history, then, to be regarded as unattainable? Are all
the grandest and most interesting problems which offer themselves to the
geological student, essentially insoluble? Is he in the position of a
scientific Tantalus--doomed always to thirst for a knowledge which he
cannot obtain? The reverse is to be hoped; nay, it may not be impossible
to indicate the source whence help will come.
In commencing these remarks, mention was made of the great obligations
under which the naturalist lies to the geologist and palaeontologist.
Assuredly the time will come when these obligations will be repaid
tenfold, and when the maze of the world's past history, through which the
pure geologist and the pure palaeontologist find no guidance, will be
securely threaded by the clue furnished by the naturalist.
All who are competent to express an opinion on the subject are, at
present, agreed that the manifold varieties of animal and vegetable form
have not either come into existence by chance, nor result from capricious
exertions of creative power; but that they have taken place in a definite
order, the statement of which order is what men of science term a natural
law. Whether such a law is to be regarded as an expression of the mode of
operation of natural forces, or whether it is simply a statement of the
manner in which a supernatural power has thought fit to act, is a
secondary question, so long as the existence of the law and the
possibility of its discovery by the human intellect are granted. But he
must be a half-hearted philosopher who, believing in that possibility,
and having watched the gigantic strides of the biological sciences during
the last twenty years, doubts that science will sooner or later make this
further step, so as to become possessed of the law of evolution of
organic forms--of the unvarying order of that great chain of causes and
effects of which all organic forms, ancient and modern, are the links.
And then, if ever, we shall be able to begin to discuss, with profit, the
questions respecting the commencement of life, and the nature of the
successive populations of the globe, which so many seem to think are
already answered.
The preceding arguments make no particular claim to novelty; indeed they
have been floating more or less distinctly before the minds of geologists
for the last thirty years; and if, at the present time, it has seemed
desirable to give them more definite and systematic expression, it is
because palaeontology is every day assuming a greater importance, and now
requires to rest on a basis the firmness of which is thoroughly well
assured. Among its fundamental conceptions, there must be no confusion
between what is certain and what is more or less probable.[1] But,
pending the construction of a surer foundation than palaeontology now
possesses, it may be instructive, assuming for the nonce the general
correctness of the ordinary hypothesis of geological contemporaneity, to
consider whether the deductions which are ordinarily drawn from the whole
body of palaeontological facts are justifiable.
[Footnote 1: "Le plus grand service qu'on puisse rendre a la science est
d'y faire place nette avant d'y rien construire."--CUVIER.]
The evidence on which such conclusions are based is of two kinds,
negative and positive. The value of negative evidence, in connection with
this inquiry, has been so fully and clearly discussed in an address from
the chair of this Society,[2] which none of us have forgotten, that
nothing need at present be said about it; the more, as the considerations
which have been laid before you have certainly not tended to increase
your estimation of such evidence. It will be preferable to turn to the
positive facts of palaeontology, and to inquire what they tell us.
[Footnote 2: Anniversary Address for 1851, _Quart. Journ. Geol. Soc._
vol. vii.]
We are all accustomed to speak of the number and the extent of the
changes in the living population of the globe during geological time as
something enormous: and indeed they are so, if we regard only the
negative differences which separate the older rocks from the more modern,
and if we look upon specific and generic changes as great changes, which
from one point of view, they truly are. But leaving the negative
differences out of consideration, and looking only at the positive data
furnished by the fossil world from a broader point of view--from that of
the comparative anatomist who has made the study of the greater
modifications of animal form his chief business--a surprise of another
kind dawns upon the mind; and under _this_ aspect the smallness of the
total change becomes as astonishing as was its greatness under the other.
There are two hundred known orders of plants; of these not one is
certainly known to exist exclusively in the fossil state. The whole lapse
of geological time has as yet yielded not a single new ordinal type of
vegetable structure.[3]
[Footnote 3: See Hooker's _Introductory Essay to the Flora of Tasmania_,
p. xxiii.]
The positive change in passing from the recent to the ancient animal
world is greater, but still singularly small. No fossil animal is so
distinct from those now living as to require to be arranged even in a
separate class from those which contain existing forms. It is only when
we come to the orders, which may be roughly estimated at about a hundred
and thirty, that we meet with fossil animals so distinct from those now
living as to require orders for themselves; and these do not amount, on
the most liberal estimate, to more than about 10 per cent. of the whole.
There is no certainly known extinct order of Protozoa; there is but one
among the Coelenterata--that of the rugose corals; there is none among
the Mollusca; there are three, the Cystidea, Blastoidea, and
Edrioasterida, among the Echinoderms; and two, the Trilobita and
Eurypterida, among the Crustacea; making altogether five for the great
sub-kingdom of Annulosa. Among Vertebrates there is no ordinally distinct
fossil fish: there is only one extinct order of Amphibia--the
Labyrinthodonts; but there are at least four distinct orders of Reptilia,
viz. the Ichthyosauria, Plesiosauria, Pterosauria, Dinosauria, and
perhaps another or two. There is no known extinct order of Birds, and no
certainly known extinct order of Mammals, the ordinal distinctness of the
"Toxodontia" being doubtful.
The objection that broad statements of this kind, after all, rest largely
on negative evidence is obvious, but it has less force than may at first
be supposed; for, as might be expected from the circumstances of the
case, we possess more abundant positive evidence regarding Fishes and
marine Mollusks than respecting any other forms of animal life; and yet
these offer us, through the whole range of geological time, no species
ordinally distinct from those now living; while the far less numerous
class of Echinoderms presents three, and the Crustacea two, such orders,
though none of these come down later than the Palaeozoic age. Lastly, the
Reptilia present the extraordinary and exceptional phenomenon of as many
extinct as existing orders, if not more; the four mentioned maintaining
their existence from the Lias to the Chalk inclusive.
Some years ago one of your Secretaries pointed out another kind of
positive palaeontological evidence tending towards the same conclusion--
afforded by the existence of what he termed "persistent types" of
vegetable and of animal life.[4] He stated, on the authority of Dr.
Hooker, that there are Carboniferous plants which appear to be
generically identical with some now living; that the cone of the Oolitic
_Araucaria_ is hardly distinguishable from that of an existing species;
that a true _Pinus_ appears in the Purbecks and a _Juglans_ in the Chalk;
while, from the Bagshot Sands, a _Banksia_, the wood of which is not
distinguishable from that of species now living in Australia, had been
obtained.
[Footnote 4: See the abstract of a Lecture "On the Persistent Types of
Animal Life," in the _Notices of the Meetings of the Royal Institution of
Great Britain_.--June 3, 1859, vol. iii. p. 151.]
Turning to the animal kingdom, he affirmed the tabulate corals of the
Silurian rocks to be wonderfully like those which now exist; while even
the families of the Aporosa were all represented in the older Mesozoic
rocks.
Among the Mollusca similar facts were adduced. Let it be borne in mind
that _Avicula, Mytilus, Chiton, Natica, Patella, Trochus, Discina,
Orbicula, Lingula, Rhynchonclla_, and _Nautilus_, all of which are
existing _genera_, are given without a doubt as Silurian in the last
edition of "Siluria"; while the highest forms of the highest Cephalopods
are represented in the Lias by a genus _Belemnoteuthis_, which presents
the closest relation to the existing _Loligo_.
The two highest groups of the Annulosa, the Insecta and the Arachnida,
are represented in the Coal, either by existing genera, or by forms
differing from existing genera in quite minor peculiarities.
Turning to the Vertebrata, the only palaeozoic Elasmobranch Fish of which
we have any complete knowledge is the Devonian and Carboniferous
_Pleuracanthus_, which differs no more from existing Sharks than these do
from one another.
Again, vast as is the number of undoubtedly Ganoid fossil Fishes, and
great as is their range in time, a large mass of evidence has recently
been adduced to show that almost all those respecting which we possess
sufficient information, are referable to the same sub-ordinal groups as
the existing _Lepidosteus, Polypterus_, and Sturgeon; and that a singular
relation obtains between the older and the younger Fishes; the former,
the Devonian Ganoids, being almost all members of the same sub-order as
_Polypterus_, while the Mesozoic Ganoids are almost all similarly allied
to _Lepidosteus_.[5]
[Footnote 5: "Memoirs of the Geological Survey of the United Kingdom.--
Decade x. Preliminary Essay upon the Systematic Arrangement of the Fishes
of the Devonian Epoch."]
Again, what can be more remarkable than the singular constancy of
structure preserved throughout a vast period of time by the family of the
Pycnodonts and by that of the true Coelacanths; the former persisting,
with but insignificant modifications, from the Carboniferous to the
Tertiary rocks, inclusive; the latter existing, with still less change,
from the Carboniferous rocks to the Chalk, inclusive?
Among Reptiles, the highest living group, that of the Crocodilia, is
represented, at the early part of the Mesozoic epoch, by species
identical in the essential characters of their organisation with those
now living, and differing from the latter only in such matters as the
form of the articular facets of the vertebral centra, in the extent to
which the nasal passages are separated from the cavity of the mouth by
bone, and in the proportions of the limbs.
And even as regards the Mammalia, the scanty remains of Triassic and
Oolitic species afford no foundation for the supposition that the
organisation of the oldest forms differed nearly so much from some of
those which now live as these differ from one another.
It is needless to multiply these instances; enough has been said to
justify the statement that, in view of the immense diversity of known
animal and vegetable forms, and the enormous lapse of time indicated by
the accumulation of fossiliferous strata, the only circumstance to be
wondered at is, not that the changes of life, as exhibited by positive
evidence, have been so great but that they have been so small.
Be they great or small, however, it is desirable to attempt to estimate
them. Let us, therefore, take each great division of the animal world in
succession, and, whenever an order or a family can be shown to have had a
prolonged existence, let us endeavour to ascertain how far the later
members of the group differ from the earlier ones. If these later
members, in all or in many cases, exhibit a certain amount of
modification, the fact is, so far, evidence in favour of a general law of
change; and, in a rough way, the rapidity of that change will be measured
by the demonstrable amount of modification. On the other hand, it must be
recollected that the absence of any modification, while it may leave the
doctrine of the existence of a law of change without positive support,
cannot possibly disprove all forms of that doctrine, though it may afford
a sufficient refutation of many of them.
The PROTOZOA.--The Protozoa are represented throughout the whole range of
geological series, from the Lower Silurian formation to the present day.
The most ancient forms recently made known by Ehrenberg are exceedingly
like those which now exist: no one has ever pretended that the difference
between any ancient and any modern Foraminifera is of more than generic
value, nor are the oldest Foraminifera either simpler, more embryonic, or
less differentiated, than the existing forms.
The COELENTERATA.--The Tabulate Corals have existed from the Silurian
epoch to the present day, but I am not aware that the ancient
_Heliolites_ possesses a single mark of a more embryonic or less
differentiated character, or less high organisation, than the existing
_Heliopora_. As for the Aporose Corals, in what respect is the Silurian
_Paloeocyclus_ less highly organised or more embryonic than the modern
_Fungia_, or the Liassic Aporosa than the existing members of the same
families?
The _Mollusca_--In what sense is the living _Waldheimia_ less embryonic,
or more specialised, than the palaeozoic _Spirifer_; or the existing
_Rhynchonelloe, Cranioe, Discinoe, Linguloe_, than the Silurian species
of the same genera? In what sense can _Loligo_ or _Spirula_ be said to be
more specialised, or less embryonic, than _Belemnites_; or the modern
species of Lamellibranch and Gasteropod genera, than the Silurian species
of the same genera?
The ANNULOSA.--The Carboniferous Insecta and Arachnida are neither less
specialised, nor more embryonic, than these that now live, nor are the
Liassic Cirripedia and Macrura; while several of the Brachyura, which
appear in the Chalk, belong to existing genera; and none exhibit either
an intermediate, or an embryonic, character.
The VERTEBRATA.--Among fishes I have referred to the Coelacanthini
(comprising the genera _Coelacanthus, Holophagus, Undina_, and
_Macropoma_) as affording an example of a persistent type; and it is most
remarkable to note the smallness of the differences between any of these
fishes (affecting at most the proportions of the body and fins, and the
character and sculpture of the scales), notwithstanding their enormous
range in time. In all the essentials of its very peculiar structure, the
_Macropoma_ of the Chalk is identical with the _Coelacanthus_ of the
Coal. Look at the genus _Lepidotus_, again, persisting without a
modification of importance from the Liassic to the Eocene formations
inclusively.
Or among the Teleostei--in what respect is the _Beryx_ of the Chalk more
embryonic, or less differentiated, than _Beryx lineatus_ of King George's
Sound?
Or to turn to the higher Vertebrata--in what sense are the Liassic
Chelonia inferior to those which now exist? How are the Cretaceous
Ichthyosauria, Plesiosauria, or Pterosauria less embryonic, or more
differentiated, species than those of the Lias?
Or lastly, in what circumstance is the _Phascolotherium_ more embryonic,
or of a more generalised type, than the modern Opossum; or a _Lophiodon_,
or a _Paloeotherium_, than a modern _Tapirus_ or _Hyrax_?
These examples might be almost indefinitely multiplied, but surely they
are sufficient to prove that the only safe and unquestionable testimony
we can procure--positive evidence--fails to demonstrate any sort of
progressive modification towards a less embryonic, or less generalised,
type in a great many groups of animals of long-continued geological
existence. In these groups there is abundant evidence of variation--none
of what is ordinarily understood as progression; and, if the known
geological record is to be regarded as even any considerable fragment of
the whole, it is inconceivable that any theory of a necessarily
progressive development can stand, for the numerous orders and families
cited afford no trace of such a process.
But it is a most remarkable fact, that, while the groups which have been
mentioned, and many besides, exhibit no sign of progressive modification,
there are others, co-existing with them, under the same conditions, in
which more or less distinct indications of such a process seems to be
traceable. Among such indications I may remind you of the predominance of
Holostome Gasteropoda in the older rocks as compared with that of
Siphonostone Gasteropoda in the later. A case less open to the objection
of negative evidence, however, is that afforded by the Tetrabranchiate
Cephalopoda, the forms of the shells and of the septal sutures exhibiting
a certain increase of complexity in the newer genera. Here, however, one
is met at once with the occurrence of _Orthoceras_ and _Baculites_ at the
two ends of the series, and of the fact that one of the simplest genera,
_Nautilus_, is that which now exists.
The Crinoidea, in the abundance of stalked forms in the ancient
formations as compared with their present rarity, seem to present us with
a fair case of modification from a more embryonic towards a less
embryonic condition. But then, on careful consideration of the facts, the
objection arises that the stalk, calyx, and arms of the palaeozoic Crinoid
are exceedingly different from the corresponding organs of a larval
_Comatula_; and it might with perfect justice be argued that
_Actinocrinus_ and _Eucalyptocrinus_, for example, depart to the full as
widely, in one direction, from the stalked embryo of _Comatula_, as
_Comatula_ itself does in the other.
The Echinidea, again, are frequently quoted as exhibiting a gradual
passage from a more generalised to a more specialised type, seeing that
the elongated, or oval, Spatangoids appear after the spheroidal
Echinoids. But here it might be argued, on the other hand, that the
spheroidal Echinoids, in reality, depart further from the general plan
and from the embryonic form than the elongated Spatangoids do; and that
the peculiar dental apparatus and the pedicellariae of the former are
marks of at least as great differentiation as the petaloid ambulacra and
semitae of the latter.
Once more, the prevalence of Macrurous before Brachyurous Podophthalmia
is, apparently, a fair piece of evidence in favour of progressive
modification in the same order of Crustacea; and yet the case will not
stand much sifting, seeing that the Macrurous Podophthalmia depart as far
in one direction from the common type of Podophthalmia, or from any
embryonic condition of the Brachyura, as the Brachyura do in the other;
and that the middle terms between Macrura and Brachyura--the Anomura--are
little better represented in the older Mesozoic rocks than the Brachyura
are.
None of the cases of progressive modification which are cited from among
the Invertebrata appear to me to have a foundation less open to criticism
than these; and if this be so, no careful reasoner would, I think, be
inclined to lay very great stress upon them. Among the Vertebrata,
however, there are a few examples which appear to be far less open to
objection.
It is, in fact, true of several groups of Vertebrata which have lived
through a considerable range of time, that the endoskeleton (more
particularly the spinal column) of the older genera presents a less
ossified, and, so far, less differentiated, condition than that of the
younger genera. Thus the Devonian Ganoids, though almost all members of
the same sub-order as _Polypterus_, and presenting numerous important
resemblances to the existing genus, which possesses biconclave vertebrae,
are, for the most part, wholly devoid of ossified vertebral centra. The
Mesozoic Lepidosteidae, again, have, at most, biconcave vertebrae, while
the existing _Lepidosteus_ has Salamandroid, opisthocoelous, vertebrae.
So, none of the Palaeozoic Sharks have shown themselves to be possessed of
ossified vertebrae, while the majority of modern Sharks possess such
vertebrae. Again, the more ancient Crocodilia and Lacertilia have vertebrae
with the articular facets of their centra flattened or biconcave, while
the modern members of the same group have them procoelous. But the most
remarkable examples of progressive modification of the vertebral column,
in correspondence with geological age, are those afforded by the
Pycnodonts among fish, and the Labyrinthodonts among Amphibia.
The late able ichthyologist Heckel pointed out the fact, that, while the
Pycnodonts never possess true vertebral centra, they differ in the degree
of expansion and extension of the ends of the bony arches of the vertebrae
upon the sheath of the notochord; the Carboniferous forms exhibiting
hardly any such expansion, while the Mesozoic genera present a greater
and greater development, until, in the Tertiary forms, the expanded ends
become suturally united so as to form a sort of false vertebra. Hermann
von Meyer, again, to whose luminous researches we are indebted for our
present large knowledge of the organisation of the older Labyrinthodonts,
has proved that the Carboniferous _Archegosaurus_ had very imperfectly
developed vertebral centra, while the Triassic _Mastodonsaurus_ had the
same parts completely ossified.[6]
[Footnote 6: As this Address is passing through the press (March 7,
1862), evidence lies before me of the existence of a new Labyrinthodont
(_Pholidogaster_), from the Edinburgh coal-field with well-ossified
vertebral centra.]
The regularity and evenness of the dentition of the _Anoplotherium_, as
contrasted with that of existing Artiodactyles, and the assumed nearer
approach of the dentition of certain ancient Carnivores to the typical
arrangement, have also been cited as exemplifications of a law of
progressive development, but I know of no other cases based on positive
evidence which are worthy of particular notice.
What then does an impartial survey of the positively ascertained truths
of palaeontology testify in relation to the common doctrines of
progressive modification, which suppose that modification to have taken
place by a necessary progress from more to less embryonic forms, or from
more to less generalised types, within the limits of the period
represented by the fossiliferous rocks?
It negatives those doctrines; for it either shows us no evidence of any
such modification, or demonstrates it to have been very slight; and as to
the nature of that modification, it yields no evidence whatsoever that
the earlier members of any long-continued group were more generalised in
structure than the later ones. To a certain extent, indeed, it may be
said that imperfect ossification of the vertebral column is an embryonic
character; but, on the other hand, it would be extremely incorrect to
suppose that the vertebral columns of the older Vertebrata are in any
sense embryonic in their whole structure.
Obviously, if the earliest fossiliferous rocks now known are coeval with
the commencement of life, and if their contents give us any just
conception of the nature and the extent of the earliest fauna and flora,
the insignificant amount of modification which can be demonstrated to
have taken place in any one group of animals, or plants, is quite
incompatible with the hypothesis that all living forms are the results of
a necessary process of progressive development, entirely comprised within
the time represented by the fossiliferous rocks.
Contrariwise, any admissible hypothesis of progressive modification must
be compatible with persistence without progression, through indefinite
periods. And should such an hypothesis eventually be proved to be true,
in the only way in which it can be demonstrated, viz. by observation and
experiment upon the existing forms of life, the conclusion will
inevitably present itself, that the Palaeozoic Mesozoic, and Cainozoic
faunae and florae, taken together, bear somewhat the same proportion to the
whole series of living beings which have occupied this globe, as the
existing fauna and flora do to them.
Such are the results of palaeontology as they appear, and have for some
years appeared, to the mind of an inquirer who regards that study simply
as one of the applications of the great biological sciences, and who
desires to see it placed upon the same sound basis as other branches of
physical inquiry. If the arguments which have been brought forward are
valid, probably no one, in view of the present state of opinion, will be
inclined to think the time wasted which has been spent upon their
elaboration.
X
GEOLOGICAL REFORM
[1869]
"A great reform in geological speculation seems now to have become
necessary."
"It is quite certain that a great mistake has been made--that British
popular geology at the present time is in direct opposition to the
principles of Natural Philosophy."[1]
[Footnote 1: On Geological Time. By Sir W. Thomson, LL.D. _Transactions
of the Geological Society of Glasgow_, vol. iii.]
In reviewing the course of geological thought during the past year, for
the purpose of discovering those matters to which I might most fitly
direct your attention in the Address which it now becomes my duty to
deliver from the Presidential Chair, the two somewhat alarming sentences
which I have just read, and which occur in an able and interesting essay
by an eminent natural philosopher, rose into such prominence before my
mind that they eclipsed everything else.
It surely is a matter of paramount importance for the British geologists
(some of them very popular geologists too) here in solemn annual session
assembled, to inquire whether the severe judgment thus passed upon them
by so high an authority as Sir William Thomson is one to which they must
plead guilty _sans phrase_, or whether they are prepared to say "not
guilty," and appeal for a reversal of the sentence to that higher court
of educated scientific opinion to which we are all amenable.
As your attorney-general for the time being, I thought I could not do
better than get up the case with a view of advising you. It is true that
the charges brought forward by the other side involve the consideration
of matters quite foreign to the pursuits with which I am ordinarily
occupied; but, in that respect, I am only in the position which is, nine
times out of ten, occupied by counsel, who nevertheless contrive to gain
their causes, mainly by force of mother-wit and common-sense, aided by
some training in other intellectual exercises.
Nerved by such precedents, I proceed to put my pleading before you.
And the first question with which I propose to deal is, What is it to
which Sir W. Thomson refers when he speaks of "geological speculation"
and "British popular geology"?
I find three, more or less contradictory, systems of geological thought,
each of which might fairly enough claim these appellations, standing side
by side in Britain. I shall call one of them CATASTROPHISM, another
UNIFORMITARIANISM, the third EVOLUTIONISM; and I shall try briefly to
sketch the characters of each, that you may say whether the
classification is, or is not, exhaustive.
By CATASTROPHISM, I mean any form of geological speculation which, in
order to account for the phenomena of geology, supposes the operation of
forces different in their nature, or immeasurably different in power,
from those which we at present see in action in the universe.
The Mosaic cosmogony is, in this sense, catastrophic, because it assumes
the operation of extra-natural power. The doctrine of violent upheavals,
_debacles_, and cataclysms in general, is catastrophic, so far as it
assumes that these were brought about by causes which have now no
parallel. There was a time when catastrophism might, pre-eminently, have
claimed the title of "British popular geology"; and assuredly it has yet
many adherents, and reckons among its supporters some of the most
honoured members of this Society.
By UNIFORMITARIANISM, I mean especially, the teaching of Hutton and of
Lyell.
That great though incomplete work, "The Theory of the Earth," seems to me
to be one of the most remarkable contributions to geology which is
recorded in the annals of the science. So far as the not-living world is
concerned, uniformitarianism lies there, not only in germ, but in blossom
and fruit.
If one asks how it is that Hutton was led to entertain views so far in
advance of those prevalent in his time, in some respects; while, in
others, they seem almost curiously limited, the answer appears to me to
be plain.
Hutton was in advance of the geological speculation of his time, because,
in the first place, he had amassed a vast store of knowledge of the facts
of geology, gathered by personal observation in travels of considerable
extent; and because, in the second place, he was thoroughly trained in
the physical and chemical science of his day, and thus possessed, as much
as any one in his time could possess it, the knowledge which is requisite
for the just interpretation of geological phenomena, and the habit of
thought which fits a man for scientific inquiry.
It is to this thorough scientific training that I ascribe Hutton's steady
and persistent refusal to look to other causes than those now in
operation, for the explanation of geological phenomena.
Thus he writes:--"I do not pretend, as he [M. de Luc] does in his theory,
to describe the beginning of things. I take things such as I find them at
present; and from these I reason with regard to that which must have
been."[2]
[Footnote 2: _The Theory of the Earth_, vol. i. p. 173, note.]
And again:--"A theory of the earth, which has for object truth, can have
no retrospect to that which had preceded the present order of the world;
for this order alone is what we have to reason upon; and to reason
without data is nothing but delusion. A theory, therefore, which is
limited to the actual constitution of this earth cannot be allowed to
proceed one step beyond the present order of things."[3]
[Footnote 3: _Ibid._, vol. i. p. 281.]
And so clear is he, that no causes beside such as are now in operation
are needed to account for the character and disposition of the components
of the crust of the earth, that he says, broadly and boldly:--" ... There
is no part of the earth which has not had the same origin, so far as this
consists in that earth being collected at the bottom of the sea, and
afterwards produced, as land, along with masses of melted substances, by
the operation of mineral causes."[4]
[Footnote 4: _Ibid._. p. 371.]
But other influences were at work upon Hutton beside those of a mind
logical by nature, and scientific by sound training; and the peculiar
turn which his speculations took seems to me to be unintelligible, unless
these be taken into account. The arguments of the French astronomers and
mathematicians, which, at the end of the last century, were held to
demonstrate the existence of a compensating arrangement among the
celestial bodies, whereby all perturbations eventually reduced themselves
to oscillations on each side of a mean position, and the stability of the
solar system was secured, had evidently taken strong hold of Hutton's
mind.
In those oddly constructed periods which seem to have prejudiced many
persons against reading his works, but which are full of that peculiar,
if unattractive, eloquence which flows from mastery of the subject,
Hutton says:--
"We have now got to the end of our reasoning; we have no data further to
conclude immediately from that which actually is. But we have got enough;
we have the satisfaction to find, that in Nature there is wisdom, system,
and consistency. For having, in the natural history of this earth, seen a
succession of worlds, we may from this conclude that there is a system in
Nature; in like manner as, from seeing revolutions of the planets, it is
concluded, that there is a system by which they are intended to continue
those revolutions. But if the succession of worlds is established in the
system of nature, it is in vain to look for anything higher in the origin
of the earth. The result, therefore, of this physical inquiry is, that we
find no vestige of a beginning,--no prospect of an end."[5]
[Footnote 5: _Ibid._, vol. i. p. 200.]
Yet another influence worked strongly upon Hutton. Like most philosophers
of his age, he coquetted with those final causes which have been named
barren virgins, but which might be more fitly termed the _hetairoe_ of
philosophy, so constantly have they led men astray. The final cause of
the existence of the world is, for Hutton, the production of life and
intelligence.
"We have now considered the globe of this earth as a machine, constructed
upon chemical as well as mechanical principles, by which its different
parts are all adapted, in form, in quality, and in quantity, to a certain
end; an end attained with certainty or success; and an end from which we
may perceive wisdom, in contemplating the means employed.
"But is this world to be considered thus merely as a machine, to last no
longer than its parts retain their present position, their proper forms
and qualities? Or may it not be also considered as an organised body?
such as has a constitution in which the necessary decay of the machine is
naturally repaired, in the exertion of those productive powers by which
it had been formed.
"This is the view in which we are now to examine the globe; to see if
there be, in the constitution of this world, a reproductive operation, by
which a ruined constitution may be again repaired, and a duration or
stability thus procured to the machine, considered as a world sustaining
plants and animals."[6]
[Footnote 6: _Ibid._, vol. i. pp. 16, 17.]
Kirwan, and the other Philistines of the day, accused Hutton of declaring
that his theory implied that the world never had a beginning, and never
differed in condition from its present state. Nothing could be more
grossly unjust, as he expressly guards himself against any such
conclusion in the following terms:--
"But in thus tracing back the natural operations which have succeeded
each other, and mark to us the course of time past, we come to a period
in which we cannot see any farther. This, however, is not the beginning
of the operations which proceed in time and according to the wise economy
of this world; nor is it the establishing of that which, in the course of
time, had no beginning; it is only the limit of our retrospective view of
those operations which have come to pass in time, and have been conducted
by supreme intelligence."[7]
[Footnote 7: _Ibid._, vol. i. p. 223.]
I have spoken of Uniformitarianism as the doctrine of Hutton and of
Lyell. If I have quoted the older writer rather than the newer, it is
because his works are little known, and his claims on our veneration too
frequently forgotten, not because I desire to dim the fame of his eminent
successor. Few of the present generation of geologists have read
Playfair's "Illustrations," fewer still the original "Theory of the
Earth"; the more is the pity; but which of us has not thumbed every page
of the "Principles of Geology"? I think that he who writes fairly the
history of his own progress in geological thought, will not be able to
separate his debt to Hutton from his obligations to Lyell; and the
history of the progress of individual geologists is the history of
geology.
No one can doubt that the influence of uniformitarian views has been
enormous, and, in the main, most beneficial and favourable to the
progress of sound geology.
Nor can it be questioned that Uniformitarianism has even a stronger title
than Catastrophism to call itself the geological speculation of Britain,
or, if you will, British popular geology. For it is eminently a British
doctrine, and has even now made comparatively little progress on the
continent of Europe. Nevertheless, it seems to me to be open to serious
criticism upon one of its aspects.
I have shown how unjust was the insinuation that Hutton denied a
beginning to the world. But it would not be unjust to say that he
persistently in practice, shut his eyes to the existence of that prior
and different state of things which, in theory, he admitted; and, in this
aversion to look beyond the veil of stratified rocks, Lyell follows him.
Hutton and Lyell alike agree in their indisposition to carry their
speculations a step beyond the period recorded in the most ancient strata
now open to observation in the crust of the earth. This is, for Hutton,
"the point in which we cannot see any farther"; while Lyell tells us,--
"The astronomer may find good reasons for ascribing the earth's form to
the original fluidity of the mass, in times long antecedent to the first
introduction of living beings into the planet; but the geologist must be
content to regard the earliest monuments which it is his task to
interpret, as belonging to a period when the crust had already acquired
great solidity and thickness, probably as great as it now possesses, and
when volcanic rocks, not essentially differing from those now produced,
were formed from time to time, the intensity of volcanic heat being
neither greater nor less than it is now."[8]
[Footnote 8: _Principles of Geology_, vol. ii. p. 211.]
And again, "As geologists, we learn that it is not only the present
condition of the globe which has been suited to the accommodation of
myriads of living creatures, but that many former states also have been
adapted to the organisation and habits of prior races of beings. The
disposition of the seas, continents and islands, and the climates, have
varied; the species likewise have been changed; and yet they have all
been so modelled, on types analogous to those of existing plants and
animals, as to indicate, throughout, a perfect harmony of design and
unity of purpose. To assume that the evidence of the beginning, or end,
of so vast a scheme lies within the reach of our philosophical inquiries,
or even of our speculations, appears to be inconsistent with a just
estimate of the relations which subsist between the finite powers of man
and the attributes of an infinite and eternal Being."[9]
[Footnote 9: _Ibid._, vol. ii. p. 613.]
The limitations implied in these passages appear to me to constitute the
weakness and the logical defect of Uniformitarianism. No one will impute
blame to Hutton that, in face of the imperfect condition, in his day, of
those physical sciences which furnish the keys to the riddles of geology,
he should have thought it practical wisdom to limit his theory to an
attempt to account for "the present order of things"; but I am at a loss
to comprehend why, for all time, the geologist must be content to regard
the oldest fossiliferous rocks as the _ultima Thule_ of his science; or
what there is inconsistent with the relations between the finite and the
infinite mind, in the assumption, that we may discern somewhat of the
beginning, or of the end, of this speck in space we call our earth. The
finite mind is certainly competent to trace out the development of the
fowl within the egg; and I know not on what ground it should find more
difficulty in unravelling the complexities Of the development of the
earth. In fact, as Kant has well remarked,[10] the cosmical process is
really simpler than the biological.
[Footnote 10: "Man darf es sich also nicht befremden lassen, wenn ich
mich unterstehe zu sagen, dass eher die Bildung aller Himmelskoerper, die
Ursache ihrer Bewegungen, kurz der Ursprung der gantzen gegenwaertigen
Verfassung des Weltbaues werden koennen eingesehen werden, ehe die
Erzeugung eines einzigen Krautes oder einer Raupe aus mechanischen
Gruenden, deutlich und vollstaendig kund werden wird."--KANT'S _Saemmtliche
Werke_, Bd. i. p. 220.]
This attempt to limit, at a particular point, the progress of inductive
and deductive reasoning from the things which are, to those which were--
this faithlessness to its own logic, seems to me to have cost
Uniformitarianism the place, as the permanent form of geological
speculation, which it might otherwise have held.
It remains that I should put before you what I understand to be the third
phase of geological speculation--namely, EVOLUTIONISM.
I shall not make what I have to say on this head clear, unless I diverge,
or seem to diverge, for a while, from the direct path of my discourse, so
far as to explain what I take to be the scope of geology itself. I
conceive geology to be the history of the earth, in precisely the same
sense as biology is the history of living beings; and I trust you will
not think that I am overpowered by the influence of a dominant pursuit if
I say that I trace a close analogy between these two histories.
If I study a living being, under what heads does the knowledge I obtain
fall? I can learn its structure, or what we call its ANATOMY; and its
DEVELOPMENT, or the series of changes which it passes through to acquire
its complete structure. Then I find that the living being has certain
powers resulting from its own activities, and the interaction of these
with the activities of other things--the knowledge of which is
PHYSIOLOGY. Beyond this the living being has a position in space and
time, which is its DISTRIBUTION. All these form the body of ascertainable
facts which constitute the _status quo_ of the living creature. But these
facts have their causes; and the ascertainment of these causes is the
doctrine of AETIOLOGY.
If we consider what is knowable about the earth, we shall find that such
earth-knowledge--if I may so translate the word geology--falls into the
same categories.
What is termed stratigraphical geology is neither more nor less than the
anatomy of the earth; and the history of the succession of the formations
is the history of a succession of such anatomies, or corresponds with
development, as distinct from generation.
The internal heat of the earth, the elevation and depression of its
crust, its belchings forth of vapours, ashes, and lava, are its
activities, in as strict a sense as are warmth and the movements and
products of respiration the activities of an animal. The phenomena of the
seasons, of the trade winds, of the Gulf-stream, are as much the results
of the reaction between these inner activities and outward forces, as are
the budding of the leaves in spring and their falling in autumn the
effects of the interaction between the organisation of a plant and the
solar light and heat. And, as the study of the activities of the living
being is called its physiology, so are these phenomena the subject-matter
of an analogous telluric physiology, to which we sometimes give the name
of meteorology, sometimes that of physical geography, sometimes that of
geology. Again, the earth has a place in space and in time, and relations
to other bodies in both these respects, which constitute its
distribution. This subject is usually left to the astronomer; but a
knowledge of its broad outlines seems to me to be an essential
constituent of the stock of geological ideas.
All that can be ascertained concerning the structure, succession of
conditions, actions, and position in space of the earth, is the matter of
fact of its natural history. But, as in biology, there remains the matter
of reasoning from these facts to their causes, which is just as much
science as the other, and indeed more; and this constitutes geological
aetiology.
Having regard to this general scheme of geological knowledge and thought,
it is obvious that geological speculation may be, so to speak, anatomical
and developmental speculation, so far as it relates to points of
stratigraphical arrangement which are out of reach of direct observation;
or, it may be physiological speculation so far as it relates to
undetermined problems relative to the activities of the earth; or, it may
be distributional speculation, if it deals with modifications of the
earth's place in space; or, finally, it will be aetiological speculation
if it attempts to deduce the history of the world, as a whole, from the
known properties of the matter of the earth, in the conditions in which
the earth has been placed.
For the purposes of the present discourse I may take this last to be what
is meant by "geological speculation."
Now Uniformitarianism, as we have seen, tends to ignore geological
speculation in this sense altogether.
The one point the catastrophists and the uniformitarians agreed upon,
when this Society was founded, was to ignore it. And you will find, if
you look back into our records, that our revered fathers in geology
plumed themselves a good deal upon the practical sense and wisdom of this
proceeding. As a temporary measure, I do not presume to challenge its
wisdom; but in all organised bodies temporary changes are apt to produce
permanent effects; and as time has slipped by, altering all the
conditions which may have made such mortification of the scientific flesh
desirable, I think the effect of the stream of cold water which has
steadily flowed over geological speculation within these walls has been
of doubtful beneficence.
The sort of geological speculation to which I am now referring
(geological aetiology, in short) was created, as a science, by that famous
philosopher Immanuel Kant, when, in 1775, he wrote his "General Natural
History and Theory of the Celestial Bodies; or an Attempt to account for
the Constitutional and the Mechanical Origin of the Universe upon
Newtonian principles."[11]
[Footnote 11: Grant (_History of Physical Astronomy_, p. 574) makes but
the briefest reference to Kant.]
In this very remarkable but seemingly little-known treatise,[12] Kant
expounds a complete cosmogony, in the shape of a theory of the causes
which have led to the development of the universe from diffused atoms of
matter endowed with simple attractive and repulsive forces.
[Footnote 12: "Allgemeine Naturgeschichte und Theorie des Himmels; oder
Versuch von der Verfassung und dem mechanischen Ursprunge des ganzen
Weltgebaeudes nach Newton'schen Grundsatzen abgehandelt."--KANT'S
_Saemmtliche Werke_, Bd. i. p. 207.]
"Give me matter," says Kant, "and I will build the world;" and he
proceeds to deduce from the simple data from which he starts, a doctrine
in all essential respects similar to the well-known "Nebular Hypothesis"
of Laplace.[13] He accounts for the relation of the masses and the
densities of the planets to their distances from the sun, for the
eccentricities of their orbits, for their rotations, for their
satellites, for the general agreement in the direction of rotation among
the celestial bodies, for Saturn's ring, and for the zodiacal light. He
finds in each system of worlds, indications that the attractive force of
the central mass will eventually destroy its organisation, by
concentrating upon itself the matter of the whole system; but, as the
result of this concentration, he argues for the development of an amount
of heat which will dissipate the mass once more into a molecular chaos
such as that in which it began.
[Footnote 13: _Systeme du Monde_, tome ii. chap. 6.]
Kant pictures to himself the universe as once an infinite expansion of
formless and diffused matter. At one point of this he supposes a single
centre of attraction set up; and, by strict deductions from admitted
dynamical principles, shows how this must result in the development of a
prodigious central body, surrounded by systems of solar and planetary
worlds in all stages of development. In vivid language he depicts the
great world-maelstrom, widening the margins of its prodigious eddy in the
slow progress of millions of ages, gradually reclaiming more and more of
the molecular waste, and converting chaos into cosmos. But what is gained
at the margin is lost in the centre; the attractions of the central
systems bring their constituents together, which then, by the heat
evolved, are converted once more into molecular chaos. Thus the worlds
that are, lie between the ruins of the worlds that have been, and the
chaotic materials of the worlds that shall be; and in spite of all waste
and destruction, Cosmos is extending his borders at the expense of Chaos.
Kant's further application of his views to the earth itself is to be
found in his "Treatise on Physical Geography"[14] (a term under which the
then unknown science of geology was included), a subject which he had
studied with very great care and on which he lectured for many years. The
fourth section of the first part of this Treatise is called "History of
the great Changes which the Earth has formerly undergone and is still
undergoing," and is, in fact, a brief and pregnant essay upon the
principles of geology. Kant gives an account first "of the gradual
changes which are now taking place" under the heads of such as are caused
by earthquakes, such as are brought about by rain and rivers, such as are
effected by the sea, such as are produced by winds and frost; and,
finally, such as result from the operations of man.
[Footnote 14: Kant's _Saemmtliche Werke_, Bd. viii. p. 145.]
The second part is devoted to the "Memorials of the Changes which the
Earth has undergone in remote Antiquity." These are enumerated as:--A.
Proofs that the sea formerly covered the whole earth. B. Proofs that the
sea has often been changed into dry land and then again into sea. C. A
discussion of the various theories of the earth put forward by
Scheuchzer, Moro, Bonnet, Woodward, White, Leibnitz, Linnaeus, and Buffon.
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